For digitally shot images, it is possible for specific imaging errors and color drifts to be corrected subsequently with the aid of suitable software. Particularly with wide angle lenses, the image corners are displayed more darkly than the middle of the image because of vignetting. The correction of such an imaging error presupposes knowledge of the camera aperture (working aperture) used in shooting.
However, digital cameras with a viewfinder eyepiece and interchangeable lenses often have no electrical or mechanical aperture interface between lens and camera that could transmit the set aperture value to the camera electronics. Moreover, the position of the aperture in the lenses differs, thus producing differences between the set and effective aperture values. Furthermore, the angle steps for aperture setting are different, or not linear meaning that different effective aperture values are to be assigned to the possibly transmitted manipulated variable.
Knowledge of the set working aperture is also important for dimensioning the intensity of a measuring flash and the flash intensity for flash shots.
A geometric measurement of the camera aperture requires that an optical measuring element be positioned in the central beam path in the camera that enables a direction of view from the camera onto the rear side of the aperture. The optical element would need to be mounted on a swiveling lever, since it has to be removed from the beam path shortly before the photoexposure. Such arrangements are known in the case of exposure measurement arrangements through the lens for analogue cameras.
In order to determine the camera aperture, the image of the lens aperture could also be imaged onto a photosensitive line through a downstream optics by a mirror element on the swiveling lever. The photosensitive line and the optics for measuring the aperture diameter could be mounted in the floor of the camera. The number of illuminated cells on the photosensitive line is proportional to the aperture diameter. The condition of a symmetrical light distribution on the line can also be considered for the purpose of calculating the aperture value.
However, it is also conceivable to provide a semitransparent mirror on the swiveling lever and to arrange the sensor for exposure measurement downstream thereof. Such arrangements are known from analogue mirror reflex cameras and enable a high level of sensitivity in exposure measurement. The mechanical outlay on swiveling mirror structures is considerable. However, the size of the imaged camera aperture can be dependent on the subject to be shot when the structures thereof have the restriction of the aperture superposed on them and simulate a smaller aperture.